1. Field of the Invention
The present invention relates to a miniature switching element that is fabricated by using MEMS technology.
2. Description of the Related Art
In the technological field of wireless communication devices such as cellular phones, a demand for miniaturization of high-frequency circuits and RF circuits has arisen in accordance with the increase in the parts that are mounted in order to implement a high performance. In order to meet such a demand, advances have been made in the miniaturization by using MEMS (micro-electromechanical systems) technology of a variety of parts constituting a circuit.
As one such part, a MEMS switch is known. The MEMS switch is a switching element in which each part is made miniature by means of MEMS technology and comprises at least a pair of contacts for executing switching through mechanical opening and closing and a drive mechanism for achieving the mechanical opening closing operation of the contact pair. MEMS switches tend to exhibit higher insulation in an open state and lower insertion loss in a closed state than switching elements made of PIN diodes and MESFETs and so forth in the switching of a GHz-order high frequency signal in particular. This is attributable to the fact that an open state is achieved by means of mechanical opening between the contact pair and to the small parasitic capacitance on account of being a mechanical switch. MEMS switches appear in Japanese Patent Application Laid Open Nos. H9-17300 and 2001-143595, for example.
FIGS. 32 and 33 show a microswitching element X6, which is an example of a conventional MEMS switch. FIG. 32 is a partial planar view of the microswitching element X6 and FIG. 33 is a cross-sectional view thereof along the line XXXIII-XXXIII in FIG. 32. The microswitching element X6 comprises a substrate 601, a fixing portion 602, a movable portion 603, a movable contact portion 604, a pair of fixed contact electrodes 605, and drive electrodes 606 and 607. The fixing portion 602 is joined to the substrate 601 and the movable portion 603 extends along the substrate 601 from the fixing portion 602. The movable contact portion 604 is provided on the underside of the movable portion 603 and the drive electrode 606 is provided over the fixing portion 602 and movable portion 603. The pair of fixed contact electrodes 605 forms a pattern on the substrate 601 so that each end faces the movable contact portion 604. The drive electrode 607 is disposed on the substrate 601 in a position corresponding to the drive electrode 606 and connected to ground. Further, a prescribed wiring pattern (not illustrated) that is electrically connected to the fixed contact electrodes 605 or drive electrode 607 is formed on the substrate 601.
When a prescribed electric potential is supplied to the drive electrode 606 of a microswitching element X6 with this constitution, an electrostatic force of attraction is produced between the drive electrodes 606 and 607. As a result, the movable portion 603 is elastically deformed to a position where the movable contact portion 604 contacts both fixed contact electrodes 605. Thus, the closed state of the microswitching element X6 is achieved. In the closed state, the pair of fixed contact electrodes 605 is electrically connected by the movable contact portion 604 and current is allowed to pass between the fixed contact electrode pair 605.
Meanwhile, when the electrostatic force of attraction acting between the drive electrodes 606 and 607 in the microswitching element X6 in the closed state ceases to exist, the movable portion 603 returns to the natural state and the movable contact portion 604 is spaced apart from the fixed contact electrodes 605. Thus, the open state of the microswitching element X6 as shown in FIG. 33 is achieved. In the open state, the pair of fixed contact electrodes 605 is electrically isolated and the passage of current between the fixed contact electrode pair 605 is prevented.
FIGS. 34 and 35 show the steps of a part of the fabrication method of the microswitching element X6. In the fabrication of the microswitching element X6, each of the fixed contact electrodes 605 and the drive electrode 607 are first formed by patterning on the substrate 601 as shown in FIG. 34A. More specifically, after a prescribed electrically conductive material is deposited on the substrate 601, a prescribed resist pattern is formed on the electrically conductive film by means of photolithography and the electrically conductive film is etched with the resist pattern serving as a mask. Thereafter, a sacrificial layer 610 is formed as shown in FIG. 34B. More specifically, a prescribed material is deposited or grown on the substrate 601 while covering the pair of fixed contact electrodes 605 and the drive electrode 607 by sputtering, for example. Thereafter, one recess 611 is formed at a point on the sacrificial layer 610 corresponding to the pair of fixed contact electrodes 605 as shown in FIG. 34C by means of etching by using a prescribed mask. Next, as shown in FIG. 34D, the movable contact portion 604 is formed by depositing a prescribed material in the recess 611 as shown in FIG. 34D.
Thereafter, as shown in FIG. 35A, a material film 612 is formed by sputtering, for example. Next, as shown in FIG. 35B, the drive electrode 606 is formed by patterning on the material film 612. More specifically, after a prescribed electrically conductive film has been deposited on the material film 612, a prescribed resist pattern is formed on the electrically conductive film by means of photolithography and etching is performed on the electrically conductive film with the resist pattern serving as a mask. Thereafter, as shown in FIG. 35C, a film body 613 that constitutes the movable portion 603 and part of the fixing portion 602 is formed by patterning the material film 612. More specifically, a prescribed resist pattern is formed on the material film 612 by means of photolithography and then the material film 612 is etched with the resist pattern serving as a mask. Thereafter, the fixing portion 602 and movable portion 603 are formed as shown in FIG. 35D. More specifically, while introducing an undercut below the movable portion 603, isotropic etching is performed on the sacrificial layer 610 via the film body 613 that functions as an etching mask so that part of the sacrificial layer 610 is residually formed as part of the fixing portion 602.
Low insertion loss in the closed state may be cited as one characteristic that is generally required of a switching element. Further, after attempting a reduction of the insertion loss of the switching element, a low electrical resistance for the pair of fixed contact electrodes is desirable.
However, in the case of the above microswitching element X6, it is difficult to establish thick fixed contact electrodes 605 and, in reality, the fixed contact electrodes 605 are thick and on the order of 2 μm. This is because of the need to secure evenness for the illustrated upper face (growth end face) of the sacrificial layer 610 that was formed temporarily in the fabrication steps of the microswitching element X6.
As mentioned earlier with reference to FIG. 34B, the sacrificial layer 610 is formed by depositing or growing a prescribed material on the substrate 601 while covering the pair of fixed contact electrodes 605. As a result, a step (not shown) that matches the thickness of the fixed contact electrodes 605 is produced on the growth end face of the sacrificial layer 610. The thicker the fixed contact electrode 605 is, the larger the step and, as the step increases, there is a tendency for the formation of the movable contact portion 604 in a suitable position and the formation of the movable portion 603 with the appropriate shape to be problematic. Further, when the fixed contact electrodes 605 are as thick as or thicker than a fixed amount, the sacrificial layer 610 that is deposited and formed on the substrate 601 sometimes breaks on account of the thickness of the fixed contact electrodes 605. When the sacrificial layer 610 breaks, it is not possible to suitably form a movable contact portion 604 or movable portion 603 on the sacrificial layer 610. Therefore, it is necessary to make the fixed contact electrodes 605 sufficiently thin so that an unreasonable step is not produced in the growth end face of the sacrificial layer 610 in the microswitching element X6. For this reason, it is sometimes difficult to implement a sufficiently low resistance for the fixed contact electrodes 605 in the microswitching element X6 and, as a result, it is sometimes impossible to implement a low insertion loss.